1. Field of the Invention
The present invention relates to a printing system that forms images on both sides of a continuous recording sheet by using a first printing apparatus, which forms an image on a first surface, and a second printing apparatus, which forms an image on a second surface, and relates to a printing apparatus and a conveyance control method.
2. Description of the Related Art
FIG. 14 is a cross-sectional view of an entire configuration of a conventional printing apparatus employed in some types of printing systems.
A web conveying mechanism of a printing apparatus P includes a web conveying unit 8 that conveys a web (roll sheet) W at a reference speed; a fixing-conveying unit 13 that nips and conveys the web W by a heat roll 13b, which is driven to rotate by a motor, and a pressing roller 13c, which is in pressure contact with the heat roll 13b so as to rotate therewith; and a buffer unit 12 that takes up slack in the web W, which is caused by a difference in conveyance between the web conveying unit 8 and the fixing-conveying unit 13.
The web W is conveyed such that the web conveying unit 8, which is located on the upstream side, first conveys the web W at the reference speed, and the heat roll 13b of the fixing-conveying unit 13, which is located on the downstream side, performs rotation control in accordance with an output of the buffer unit 12 and conveys the web W so as to follow the reference speed while applying heat and pressure to the web W to fix a toner image formed thereon.
When the web conveyance is to be stopped, a stop timing of the fixing-conveying unit 13 is changed in accordance with an output of a buffer-amount sensor 12b, so that a buffer position at the time of stop of the web conveyance reaches a specific stop position.
As a conventional stop control method, a technology has been proposed in which a brake amount is adjusted in accordance with change in the conveying amount of the heat roll 13b per tiny unit of time while the heat roll 13b is decelerating for stopping printing (see, for example, Japanese Patent Application Laid-open No. 2007-316411).
As for a brake force on the heat roll 13b, there has been known a method of applying a reverse voltage on a motor that is used as a driving source of the heat roll 13b or a method of using a brake force that is generated by directly applying a load on the heat roll 13b. Further, a friction load that occurs at a contact portion of the web W in the printing apparatus may be used as a brake force. In order to reduce influence of the friction load on the web W, variation in a braking distance at the time of stop of the web conveyance is reduced by increasing a reverse voltage applied to a motor as a brake force or increasing a load directly applied to the heat roll 13b. 
However, when the web conveyance speed is increased, an inertial force in a conveying direction of the web W increases. In this case, if the brake force remains great, stress to strain on the heat roll 13b increases and the heat roll 13b is easily torn. Therefore, it is necessary to lower the brake force with increase in the speed.
In recent years, however, various webs made of various materials are used and the brake force on the heat roll 13b greatly varies depending on the materials or a ream weight of the web W. Therefore, the rate of stopping the heat roll 13b by the friction load applied by the web W is increasing, so that a difference in a braking distance at the time of stop of the web conveyance may occur.
When duplex printing is performed on the web W, another printing apparatus P is serially disposed on the subsequent stage of the printing apparatus P such that the printing apparatus P on the preceding stage performs printing on a front side of the web W and the printing apparatus P on the subsequent stage performs printing on a back side of the web W. Therefore, oil applied to the heat roll 13b in the fixing-conveying unit 13 adheres to the web W and the web W with the oil is conveyed to the printing apparatus P on the subsequent stage. Consequently, a difference in a frictional force occurs due to the oil adhered to the web W and a braking distance may vary in the printing apparatus P on the subsequent stage.
However, when variation in the braking distance of the web W increases as above, and if a web conveying amount of the fixing-conveying unit 13 at the time of stop of the web conveyance is decreased, the buffer amount between the web conveying unit 8 and the fixing-conveying unit 13 increases, leading to slack in the web W on a buffer plate 12a. When the web conveying amount of the fixing-conveying unit 13 is increased, the buffer amount between the web conveying unit 8 and the fixing-conveying unit 13 decreases, so that the web W may be overly pulled toward the fixing-conveying unit 13 side and may be torn, which is a problem.
In particular, in the above-mentioned system in which another printing apparatus P is serially disposed on the subsequent stage of the printing apparatus P in order to perform duplex printing on the web W such that the printing apparatus P on the preceding stage performs printing on the front side of the web W and the printing apparatus P on the subsequent stage performs printing on the back side of the web W, because oil applied to the heat roll 13b in the fixing-conveying unit 13 adheres to the web W and the web W with the oil is conveyed to the printing apparatus P on the subsequent stage, a difference in a frictional force occurs due to the oil adhered to the web W in the printing apparatus P on the subsequent stage. Therefore, variation in the braking distance increases and the web W may be frequently torn, which makes it difficult to smoothly perform printing operations.